Sending Faxes over digital connections to the PSTN (Public Switched Telephone Network) or over digital buses is not straight forward. Where an analog Fax machine connects to the PSTN via an analog interface that is connected to a digital telecommunication line (like ISDN, T1, E1, DS3, E3, fiber etc.) the fax reception/transmission is not reliable, because in general the digitization of the FAX signal is driven by a different clock frequency from the synchronized clock frequency of the digital network. Because the sampling frequency of the FAX stream by an analog interface is slightly higher or lower than the clock frequency of the digital line, data loss occurs resulting in imperfect faxes. If the PSTN connection is also analog, unless both the port supporting the FAX machine and the analog PSTN connection are completely synchronized, the same issue of imperfect faxes arises. This is a well known problem.
Failure to synchronize incoming and outgoing signals affects the quality of connection (voice in regular voice transmission). In the case of Fax transmission the quality of the connection becomes a problem. FAXes will exhibit regular errors, and in extreme cases will fail entirely.
The same problem can be manifested when the system that is connecting to one or more Fax machines uses a High-Speed serial interface (like Ethernet, PCI express, RapidI/O etc.)
One possible solution is to send each Fax as a complete packet (not real time) and buffer it before transmission to the Fax machine. But this solution is not free from limitations. It doesn't allow operating in real time and can not support long faxes. There is a possibility to use special methods of transmission like T38, but this implies an IP network and requires significant encoding/decoding equipment.
Lack of synchronization will affect voice quality. However, voice distortion can be compensated by data interpolation in case of overruns and signal repetition in case of under runs errors.
The modern telephone system includes the concept of a PBX (Private Branch eXchange) for connection among the internal telephones of a private organization and PSTN.
The PBX can have various numbers of analog interfaces to the local analog phones and local fax machines.
The configuration of the PBX has to be flexible (for supporting different numbers and types of telephones/faxes in the organization), and consequently the many popular PBXs are based on regular personal computers (PCs) and use specific add-in telephony interface boards both analog and digital) to connect to the analog telephone lines of the local phones and Faxes.
The modern standard PC allows the creation of PBX designs with almost unlimited flexibility, but the resources of a single PC can be easily exhausted by handling such CPU intensive processes as ECHO cancellation, voice compression, implementing conference calling, voice messaging, call recording etc. In this case the PBX function can be implemented based on more than one PC. Said PCs can be connected via High-Speed interface (like Ethernet, PCI express, RapidI/O etc.).
The well known synchronous telecommunication interface “H.100” was designed to allow connections between different telecommunication equipment and supports data transfer in particular between analog interface add-in cards that will support the connection of Fax machines, and digital telecommunication add-in cards. By using the H.100 interface all add-in telephony interface boards can be synchronized. The data is transmitted via synchronous parallel bus (H.100 interface bus) in a daisy chain. However, H.100 is intended only for telecommunication data transmission, and it cannot be used as main computer bus. So the H.100 bus is an additional interface bus to the main computer high speed interface bus (like PCI, PCI-X, PCI Express etc.).
By using this H.100 bus between telephony cards the voice or Fax traffic will move between analog an digital add-in boards synchronously, and the entire system will be driven by a single clock source typically derived from the PSTN master clock. This prevents buffer overrun or underrun conditions, and in this case there will be no problems with voice quality and fax reception.
However, this solution has two drawbacks. One is that while all synchronous data traffic is moving via H.100 bus from one interface to another, the system can not use the resources of the main PC processor for voice processing. The alternative is to create data processing units on the telephony interface cards themselves or install special data processing boards with connection to the H.100 bus. Of course this solution immediately increases the system cost and reduces flexibility of the PBX, by wasting the resources of the PC.
The second problem is that H.100 bus has a limited maximum physical length of the bus (the maximum length is 20 inches) and a limited number of boards that can be connected to it (the maximum is 20 boards). Because of those limitations the solution using the H.100 bus is not practical for PBXs based on multiple computers.
U.S. Pat. No. 7,283,270 of Boire-Lavigne S., et al presents a FAX transmission system joining the PSTN and IP Network using T.30, T.38, H.323 etc. interfaces. T.38 interface includes data compression (and decompression) so as to have an opportunity of repeating transmitted messages to correct possible errors. Such procedures seriously load communication lines and the apparatus, reducing system performance.
U.S. Pat. No. 6,721,282 of Mothey C. F. relates to telecommunication apparatus that is adapted to extend voice channel capacity and also uses compressing/decompressing procedures. For this purpose proposed apparatus includes special DSP modules for data processing.
In U.S. Pat. No. 6,552,826 Adler B. M., et al. the facsimile transmission system uses a buffer node including buffer storage for temporary storing at least a portion of received fax transmission and decompression device and an intermediate node including compression device.
In Pat. Appl. 20060109503 the author Seung-wook H. proposed a method (and system) for transmitting fax data to an external device (fax machine), that requires determining that an error has occurred in the fax transmission data and re-transmitting the fax data to the external device in response to the request signal.
Thus, general PBX based on the more then one computer, require for the reception/transfer voice and Fax data significant additional equipment (H.100 interface and supported data processing circuits), or special dedicated equipment for data compression/decompression. This invention is intended to eliminate these drawbacks of existing PBX systems by allowing for synchronization of analog and digital PSTN connections, and hence error free faxing and modem connections.